PROJECT SUMMARY: Our proposed comprehensive model of DNA radiation-induced damage describes physicochemical events from the initial track structure and DNA ion-radical-excited state formation through hole and electron transfer, to chemical events involving free radical processes that lead to secondary radicals and, finally, to combination and redox processes that result in DNA damage such as base and sugar damage, strand scission and associated base release. The overall goal of the current proposed effort is to test several aspects this overall model, to modify it as appropriate, and thereby elucidate fundamental mechanisms of radiation damage to DNA by radiations of varying linear energy transfer (LET). These studies will be performed under conditions that emphasize the direct effect of radiation, and will employ magnetic resonance spectroscopies (ESR, NMR), product analyses (HPLC, LC- MS/MS), gamma and cyclotron heavy ion-beam irradiation, as well as theoretical modeling including time dependent density functional theory (TD-DFT). The first aim will determine the protonation state of the hole (guanine cation radical (G?+)) in a G-quadruplex. Subsequently, it will test the feasibility of hole transfer process from a single guanine base to a guanine in G-quadruplexes within a DNA-oligomer as well as the hole transfer process from a G-quadruplex to DNA base analogs of varying redox potential such as the easily oxidized pseudoisoguanine (PIG). It will also whether G-quadruplex is an ultimate local sink of radiation-produced electrons. These studies will test the hypotheses that the G-quadruplex, owing to its low redox potential, will protect against radiation-induced hole and electron transfer processes as well as excitation events and are of significance to radiation damage to telomeres. The second aim involves the role of ion-beams in DNA damage in nucleohistone and in DNA and will test the hypothesis that the yield of sugar radicals from LEE and excited state processes increase as LET increases along the beam path. C3??dephos will be used as a marker for LEE-induced processes along the beam path. The ratio of purine to pyrimidine base release will be used as a marker for excitation- induced strand breaks. We will test the hypotheses that at the Bragg peak, where fewer radicals are stabilized in nucleohistone or in DNA owing to the ion-radical recombination events, changes will occur in the individual DNA damage product yields and their nature from that found in the ion-beam LET plateau region. The third aim will employ theoretical calculations to further test and confirm molecular mechanisms proposed in each of the above-mentioned aims. In addition, several hypotheses will be tested. The most important is that redox properties calculated with DFT can predict the fate of DNA- radical intermediates. This aim directly aids Aim 1. These efforts will allow us to establish new insights into the fundamental radiation-induced physicochemical processes which are important for biomedical research.